<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>

</head>
<body>
  <div id="app">
    <div :class="a">111</div>
    <div :class="[a,'box2']">22</div>
    <div :class="{active: 1==1, b: false, c: 1<3}">33</div>
    <div :class="[a,'box2', {active: 1==1, b: false, c: 1<3}]">44</div>
  </div>
  <script src="./vue.js"></script>
  <script>
    const vm = new Vue({
      el: '#app',
      data: {
        a: 'box'
      }
    })
    
  </script>
</body>
</html>